Read an Academic Passage

[1]People on Earth always see the same side of the Moon. This is not because the Moon does not rotate. In fact, it turns once on its axis in exactly the time it takes to orbit Earth, about 27 days. Astronomers call this match tidal locking, and it is common among the moons of the solar system.

[2]Tidal locking results from gravity. Earth pulls more strongly on the near side of the Moon than on the far side, stretching the Moon slightly into an egg shape with two bulges. Long ago, when the Moon rotated faster, these bulges were constantly being pulled out of line with Earth. The friction this created inside the Moon gradually slowed its spin until the bulges stayed pointed toward Earth, and from then on the Moon kept one face toward its planet.

[3]The same process is acting on Earth, though much more slowly, because Earth is far larger than the Moon. The Moon's gravity raises tides in Earth's oceans, and the friction of these tides slows Earth's spin very slightly: each century, one full rotation takes roughly two thousandths of a second longer than before. Over many billions of years, this effect could in theory lock Earth so that one side always faces the Moon.